fix null pointer dereference in screen_bottom_address() in librz/core…

[Syh-Xiong]

…/tui/visual.c

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Detailed description
I think when realloc fails to allocate and returns NULL into rtn, it will cause the subsequent memmove(rtn + 2, rtn, addr_len) to dereference a null pointer.

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Test plan

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Closing issues

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.36%. Comparing base (4b103d1) to head (e6bdea5).

Files with missing lines	Patch %	Lines
librz/core/tui/visual.c	0.00%	6 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
librz/core/tui/visual.c	13.32% <0.00%> (-0.03%)	⬇️

... and 13 files with indirect coverage changes

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